Compliant hydrodynamic gas bearings are ideally suited to conditions found in high performance machinery subject to extreme conditions of temperature and speed. Such bearings are not subject to the significant operational and durability limitations characteristic of bearings which require liquid lubrication and complex lubricant, support, cooling and sealing systems.
While known compliant hydrodynamic gas bearings are satisfactory in many applications, there is room for improvement in overall load capacity and dynamic stability of the rotor-bearing system at all speeds.
The principal difficulties experienced in known compliant foil bearing systems has been that known bearings exhibit limited foil stiffness resulting in an inability to control oscillatory motions between the movable and stationary members at certain critical bearing speeds. While it is advantageous to minimize the thickness of the foil bearing in order to render it sufficiently compliant to conform to the supported member uniformly under all conditions, such thin foils exhibit a reduced load-bearing capacity.
Another problem relates to oscillation under actual load conditions. The shaft in a high-speed radial bearing tends to orbit about the geometric center of the bearing support and the amplitude of the oscillation is maximized at certain critical speeds. In order to control this oscillation, it is desirable to provide substantial damping in the bearing assembly. This problem is especially critical in the case of small journal bearings in which only limited space is available for the bearing assembly. Proper control or elimination of high-speed instability will permit the bearing to operate to the burst speed of the rotating assembly.
There are two principal types of instability, the first of which is known as "synchronous whirl" and the second of which is known as "half-speed whirl". During relatively low-speed rotation of a shaft, the orbiting motion of the geometric center of the shaft about the geometric center of the bearing support tends to set up centrifugal forces acting on the shaft which cause the shaft to orbit or whirl at a rotatioal speed equal to the rotational speed of the shaft about its own axis. This orbiting or whirling motion is synchronous whirl and occurs at the lowest critical speed of the bearing.
Half-speed whirl is a more serious instability which occurs as the shaft approaches a speed approximately equal to twice its lowest critical speed. At twice critical speed the shaft inherently tends to undergo harmonic vibration at its lowest critical frequency. This harmonic vibration is superimposed upon the synchronous whirl and is stimulated or excited by the load carrying rotating fluid wedge whose average velocity about the shaft now approaches the lowest critical speed. As a result, orbital excursions of the shaft rapidly increase in amplitude. During half-speed whirl the whirl velocity of the shaft approximates the average velocity of the fluid wedge. When this occurs the speed of the fluid wedge relative to the orbiting shaft tends toward zero, causing a loss of fluid film support. Since the shaft is operating at a relatively high speed, contact between the shaft and bearing may cause damage or failure of the bearing.
Compliant foil bearings which accommodate orbital excursions of the shaft while providing a cushioning and dampening effect have been found to greatly reduce whirl instability. However, known bearings do not present a complete solutions to the problems of hydrodynamic bearings since there is till a need for greater load-carrying capacity while providing improved whirl stability. Moreover, there is a need for a bearing which compensates for misalignment between the movable and stationary elements. Additionally, thermal distortion of the movable element due to rapid heating of the surface of the movable element nearest the stationary element while the remainder of the movable element remains relatively cool remains a problem. The temperature gradient imposed tends to distort the uniform surface of the movable element.